The field of heat exchanger designs is replete with applications of fins to improve the heat transfer. In some embodiments, this is heat transfer by forced convection mechanism. The heat transfer by forced convection takes place between a solid surface and fluid in motion, which may be gas or liquid, and it comprises the combined effects of conduction and fluid flow. This type of heat transfer occurs in most of the conventional heating systems, either hot water or electric and industrial heat exchangers.
In the cracking process of a paraffin, such as ethane or naphtha, feed flows through a furnace coil (pipe) that is heated up to 1050° C. inside the radiant section of a cracking furnace. At these temperatures, the feed undergoes a number of reactions, including a free radical decomposition (cracking), reformation of a new unsaturated product and the coproduction of hydrogen. These reactions occur over a very short period of time that corresponds to the feed residence time in a coil.
The interior of the radiant section of the furnace is lined with heat absorbing/radiating refractory and is heated, for example, by gas fired burners. The heat transfer within the furnace, between flame, combustion gases, refractory and the process coils is mostly by radiation, and also by forced convection.
There is a drive to improve the efficiency of cracking furnaces as this reduces process costs and greenhouse gas emissions. There have been two main approaches to improving efficiency; improving heat transfer to the furnace coils, i.e., from flame, combustion gases and refractory walls to the external surface of a process coil, and improving heat transfer within the coil, i.e., from the coil walls into the feed flowing inside the coil.
One of the methods representing the second approach, is the addition of internal fins to the inner walls of the furnace coil, to promote the “swirling” or mixing of the feed within the coil. This improves the convective heat transfer from the coil walls to the feed as the turbulence of the feed flow is increased and the heat transferring surface of the hot inner wall of the coil is increased as well.
U.S. Pat. No. 5,950,718 issued Sep. 14, 1999 to Sugitani et al., assigned to Kubota Corporation provides one example of this type of technology.
The papers “Three dimensional coupled simulation of furnaces and reactor tubes for the thermal cracking of hydrocarbons”, by T. Detemmerman, G. F. Froment, (Universiteit Gent, Krijgslaan 281, b9000 Gent—Belgium, mars-avri, 1998); and “Three dimensional simulation of high internally finned cracking coils for olefins production severity”, by Jjo de Saegher, T. Detemmerman, G. F. Froment, (Universiteit Gent1, Laboratorium voor Petrochernische Techniek, Krijgslaan 281, b-9000 Gent, Belgium, 1998 provide a theoretical simulation of a cracking process in a coil which is internally finned with helicoidal and longitudinal fins (or, rather, ridges or bumps). The simulation results are verified by lab scale experiments, where hot air flows through such internally finned tubes. The papers conclude that the tube with internal helicoidal fins performs better then with internal longitudinal fins and that the results for “a tube with internal helicoidal fins are in excellent agreement with industrial observations”. However, no experimental data are provided to support these conclusions. There is also no comparison made to the performance of a bare tube, with no internal ribs or fins. The authors agree that one potential disadvantage of such coils with internal fins is that carbon deposits may build up on the fins, increasing the pressure drop through the tube.
U.S. Pat. No. 3,476,180 issued Nov. 4, 1969 to Straight Jr. et al., assigned to Esso Research and Engineering Company teaches tubes for use in the convection section of a cracking furnace. There are pins that are on the surface of the downward face of the tubes in the convection section of the furnace. The pins are tightly packed and there is no dimension given for the length of the pin. In the convection section of the furnace, the feed is relatively cool. The heat loss from the pins is low. In the radiant section of the furnace, it may be necessary to limit the length of the pin or the pin may become a radiator, in effect, dissipating heat from the tube. The patent fails to suggest the subject matter of the present claims.
U.S. Pat. No. 5,437,247 issued Aug. 1, 1995 to Dubil et al., assigned to Exxon Research and Engineering Company teaches elongated plates pivotably mounted on at least one horizontal tube in a vertical row that is one row removed from the wall. When the plate is pivoted down, it prevents channeling of the hot gases through the convection section of the furnace (flue).
Canadian Patent No. 1,309,841 issued Aug. 25, 1988 to Fernandez-Baujin et al., assigned to Lummus Crest Inc., USA teaches putting “studs” (“pins”) on the external and internal surfaces of pyrolysis coils used in the radiant section of a cracker. The “pins” are not arranged in longitudinal rows. Additionally, the “pins” have a length from 0.5 to 0.75 inches. This is longer than the pin lengths disclosed herein.
U.S. Pat. No. 6,250,340 issued Jun. 26, 2001 to Jones et al., assigned to Doncasters PLC, teaches pipes for chemical reactions, such as, furnace tubes having internal grooves
The report High Efficiency, Ultra low emissions, Integrated Process Heater by TIAX LLC of Jun. 19, 2006 to the U.S. Department of Energy, Golden Field Office discloses furnace tubes having studs 2 inches long and 0.5 inches in diameter. The studs were placed in longitudinal arrays on the side of the furnace tube facing the refractory wall. The studs have a length of 2 inches (page 3-26). This teaches away from the subject matter disclosed herein.
In some embodiments, the present invention seeks to provide an enhanced heat transfer, comparable to that of a fin, while reducing the stress on the tube or pipe.